1. Technical Field
This invention relates to methods and apparatuses for producing thin films and nanoparticles deposits, and in particular, for producing thin films and nanoparticle deposits by electrostatic spraying of nanodrops.
2. Background Art
Electrostatic spraying apparatuses are known in the art. See U.S. Pat. No. 5,344,676, the disclosure of which is incorporated herein by reference. In an electrostatic spraying apparatus, electric charge is supplied to a surface of a liquid. When the repulsive forces within the liquid caused by the electric charge exceed the surface tension maintaining the surface of the liquid, the surface of the liquid is explosively disrupted to form small jets. The small jets break up into streams of charged liquid clusters referred to as nanodrops (liquid phase) or nanoparticles (solid phase formed by solidifying nanodrops).
The resulting stream of nanodrops can then be directed onto a surface of a target material or substrate. Over time, the nanodrops will collect on the surface of the target material to form a thin film on the surface.
This electrostatic method, while an improvement over other conventional methods of thin film fabrication, such as chemical vapor deposition, sputtering, laser ablation, and spray pyrolysis, may have significant disadvantages. For one thing, the explosive disruption of the surface of the liquid forms nanodrops which are small, but which may move at high velocities. As a consequence, when the nanodrops collide with the target material, a great deal of momentum may be transferred from the nanodrops to the target material. Where the target material is being suspended, for example by use of acoustical pressure fields, the transfer of momentum from the nanodrops to the target material may force the target material out of alignment with the supporting fields.
Furthermore, as nanodrops collect on the surface of an electrically insulated target material, a space charge problem may occur. Because the electrically insulated target material cannot efficiently transport charge away from the surface, certain areas of the surface may assume the charge of the nanodrops which have been applied to the surface. As a consequence, the charged surface may affect the further application of nanodrops to the surface. As a further consequence, a non-uniform film may result on the surface.